The present invention relates to the field of making hollow articles, and in particular bottles or receptacles, which are obtained by stretching and blowing a parison. The invention relates mainly to improvements to the thread on the neck of a parison based on at least one aromatic polyester resin, and also to a receptacle obtained by stretching and blowing such a parison, with the neck of the receptacle being made stable with respect to heat by being subjected to a thermal stabilization operation. The invention is advantageously applicable in the manufacture of receptacles and in particular bottles made of polyester and in particular of polyethylene terephthalate (PET), which bottles can be filled with a liquid at high temperature; for example, this may relate to receptacles that are filled with a pasteurized beverage while it is at high temperature, or to receptacles that are reusable and that can be washed in hot water.
Most polyester receptacles, and in particular PET-based bottles, are closed by means of a removable cap that can be fitted to the neck of the bottle by being screwed thereon. To this end, the neck of the parison that is subsequently to form the bottle is provided with a thread that matches the inside thread of the closure cap. Industrially speaking, receptacles are closed on a manufacturing line in automatic manner after the receptacles have been filled. Each receptacle is closed by automatically applying a determined amount of tightening torque to the cap while it is on the receptacle.
Obtaining very good sealing of receptacles by means of their caps is a constant concern of manufacturers and of bottlers, particularly when the receptacles are intended to contain an aerated beverage. Any leakage through the cap significantly reduces the lifetime of the beverage inside the receptacle, with the carbon dioxide contained in the beverage escaping more easily and more quickly to the outside while, on the contrary, oxygen penetrates more easily to the inside of the receptacle. To close the receptacle in sealed manner, it is essential for the thread on the neck of the bottle to match as closely as possible the inside thread of the cap. In practice, it is particularly important for the profile, in longitudinal section, of the thread on the receptacle to match the profile of the inside thread of the cap, i.e., given the types of cap presently available on the market, the profile must be symmetrical. The greater the asymmetry in longitudinal section of the profile of the thread, the higher the risk of the sealing via the cap being defective. On the filling line and while the cap is being put into place on the receptacle, there is also a risk that the controlled torque is caused to be wrong so that the cap is not, in fact, tightened sufficiently. For these reasons, manufacturers of receptacles made of polyester, and in particular bottles based on PET, have made use of molds that enable parisons to be made by injection molding in which the thread has a profile in longitudinal section that is as symmetrical as possible.
Another problem with receptacles based on a polyester resin such as PET lies in their poor thermal stability which makes them too easily deformable when exposed to high temperatures, for example when they are filled with a liquid at high temperature such as a pasteurized beverage, or when they are immersed in a hot washing liquid. To mitigate that drawback, a technique commonly used at present is to subject such receptacles during manufacture to an operation referred to as xe2x80x9cthermal stabilizationxe2x80x9d which consists in raising them for a predetermined length of time to a high temperature and in practice to a temperature that is higher than the temperature to which the receptacles might subsequently be subjected in use, and this has the consequence of increasing the degree of crystallinity of the resin by a thermal effect. This operation can be performed on the manufacturing line directly on the parison, or on the final receptacle as obtained after final stretching and blowing of the parisons, or indeed on the article at an intermediate stage between the parison and the finally stretched and blown receptacle. The thermal stabilization can be performed, for example, by passing the articles through a xe2x80x9ccrystallizingxe2x80x9d oven fitted with infrared lamps. In practice, polyester receptacles which are thermally stabilized during manufacture are easily recognized because they are opaque in the neck, due to the increased degree of crystallinity of the resin, and in particular to the increase in the size and number of spherolites in the crystal structure.
Thermally stabilizing the neck of a receptacle made on the basis of at least one polyester resin, and in particular of PET, advantageously serves to reduce very considerably any phenomenon whereby the profile of the thread on the neck of the receptacle deforms in the event of the thread subsequently being exposed to a high temperature, for example by being filled with a hot liquid. Thus, e.g. for the purpose of enabling the receptacle to be reused, it is possible for it to be washed at a higher temperature, and in particular a temperature close to the vitreous transition temperature of the material from which the receptacle is made, while leaving substantially unchanged the degree of sealing that can be obtained at the cap of the receptacle. The drawback is that the operation of thermally stabilizing the receptacle, itself deforms the profile of the thread, thereby reducing the symmetry of the profile compared with the initial profile that the thread had previously at the parison stage on leaving the injection mold. This deformation of the thread under the effect of heat is further increased when the thread is of the discontinuous type, i.e. when it is made up of a succession of segments separated by longitudinal grooves that serve as vents to allow the gas contained in the receptacle to be released progressively when it is opened.
Solutions proposed in the past for mitigating deformation of the thread when the parison or the bottle is subjected to heat treatment have all sought to make the thread better at withstanding deformation under the effect of heat. A solution of that type is proposed, for example, in European patent application EP 0 675 047, which teaches making a PET parison whose neck thread is of the discontinuous type, and in which the grooves between successive segments of the thread are characteristically distributed at regular intervals around the circumference of the neck of the bottle. Nevertheless, that solution suffers from the drawback of very considerably complicating the structure of the mold for injecting the parison, and, in practice, makes the method too difficult to implement.
The invention seeks to propose a novel solution to the problem of the thread on the neck of a parison or of a receptacle becoming deformed, as happens when the neck is based on at least one aromatic polyester resin, and in particular when it is based on PET, and is subjected to an operation of thermal stabilization.
Unlike solutions proposed in the past, the invention consists not in making the thread less deformable when the neck is subjected to an operation of thermal stabilization, but on the contrary in making use of the deformation of the thread under the effect of heat, by anticipating it.
Thus, the invention mainly provides a parison made from one or more thermoplastic resins and including a neck based on an aromatic polyester, which neck is provided with a thread that has not been thermally stabilized. In characteristic manner, the thread has a profile in longitudinal section that is asymmetrical, thereby making it possible to obtain a thread whose profile in longitudinal section presents improved symmetry after the neck has been thermally stabilized.
The invention also provides a receptacle, and in particular a bottle based on PET which can be filled with a hot liquid and, more particularly, which can be reused, the receptacle being obtained by stretching and blowing the above-specified parison, and then subjecting the neck Thereof to thermal stabilization.